NOx emissions from diesel engines are an environmental problem. Several countries, including the United States, have long had regulations pending that will limit NOx emissions from trucks and other diesel-powered vehicles. Manufacturers and researchers have put considerable effort toward meeting those regulations.
In gasoline powered vehicles that use stoichiometric fuel-air mixtures, three-way catalysts have been shown to control NOx emissions. In diesel-powered vehicles, which use compression ignition, the exhaust is generally too oxygen-rich for three-way catalysts to be effective.
Several solutions have been proposed for controlling NOx emissions from diesel-powered vehicles. One set of approaches focuses on the engine. Techniques such as exhaust gas recirculation and partially homogenizing fuel-air mixtures are helpful, but these techniques alone will not eliminate NOx emissions. Another set of approaches remove NOx from the vehicle exhaust. These include the use of lean-burn NOX catalysts, selective catalytic reduction (SCR), and lean NOX traps (LNTs).
Lean-burn NOx catalysts promote the reduction of NOx under oxygen-rich conditions. Reduction of NOx in an oxidizing atmosphere is difficult. It has proven challenging to find a lean-burn NOx catalyst that has the required activity, durability, and operating temperature range. Lean-burn NOx catalysts also tend to be hydrothermally unstable. A noticeable loss of activity occurs after relatively little use. Lean-burn NOx catalysts typically employ a zeolite wash coat, which is thought to provide a reducing microenvironment. The introduction of a reductant, such as diesel fuel, into the exhaust is generally required and introduces a fuel economy penalty of 3% or more. Currently, peak NOx conversion efficiencies for lean-burn NOx catalysts are unacceptably low.
SCR generally refers to selective catalytic reduction of NOx by ammonia. The reaction takes place even in an oxidizing environment. The NOx can be temporarily stored in an adsorbent or ammonia can be fed continuously into the exhaust. SCR can achieve high levels of NOx reduction, but there is a disadvantage in the lack of infrastructure for distributing ammonia or a suitable precursor. Another concern relates to the possible release of ammonia into the environment.
To clarify the state of a sometime ambiguous nomenclature, it should be noted that in the exhaust aftertreatment art, the terms “SCR catalyst” and “lean NOx catalyst” are occasionally used interchangeably. Where the term “SCR” is used to refer just to ammonia-SCR, as it often is, SCR is a special case of lean NOx catalysis. Commonly when both types of catalysts are discussed in one reference, SCR is used with reference to ammonia-SCR and lean NOx catalysis is used with reference to SCR with reductants other than ammonia, such as SCR with hydrocarbons.
LNTs are devices that adsorb NOx under lean exhaust conditions and reduce and release the adsorbed NOx under rich condition. A LNT generally includes a NOx adsorbent and a catalyst. The adsorbent is typically an alkaline earth compound, such as BaCO3 and the catalyst is typically a combination of precious metals, such as Pt and Rh. In lean exhaust, the catalyst speeds oxidizing reactions that lead to NOx adsorption. In a reducing environment, the catalyst activates reactions by which adsorbed NOx is reduced and desorbed. In a typical operating protocol, a reducing environment will be created within the exhaust from time-to-time to regenerate (denitrate) the LNT.
A LNT can produce ammonia during denitration. Accordingly, it has been proposed to combine a LNT and an ammonia-SCR catalyst into one system. Ammonia produced by the LNT during regeneration is captured by the SCR catalyst for subsequent use in reducing NOx, thereby improving conversion efficiency over a stand-alone LNT with no increase in fuel penalty or precious metal usage. U.S. Pat. No. 6,732,507 describes such a system. U.S. Pat. Pub. No. 2004/0076565 describes such systems wherein both components are contained within a single shell or disbursed over one substrate.
In addition to accumulating NOx, LNTs accumulate SOx. SOx is the combustion product of sulfur present in ordinarily fuel. Even with reduced sulfur fuels, the amount of SOx produced by combustion is significant. SOx adsorbs more strongly than NOx and necessitates a more stringent, though less frequent, regeneration. Desulfation requires elevated temperatures as well as a reducing atmosphere. The temperature of the exhaust can be elevated by engine measures, particularly in the case of a lean-burn gasoline engine, however, at least in the case of a diesel engine, it is often necessary to provide additional heat. Typically, this heat is provided through the same types of reactions as used to remove excess oxygen from the exhaust. The temperature of the LNT is generally controlled during desulfation as the temperatures believed to be required for desulfation are generally close to those at which the LNT catalyst undergoes thermal deactivation.
Creating a reducing environment for LNT regeneration involves eliminating most of the oxygen from the exhaust and providing a reducing agent. Except where the engine can be run stoichiometric or rich, a portion of the reductant reacts within the exhaust to consume oxygen. The amount of oxygen to be removed by reaction with reductant can be reduced in various ways. If the engine is equipped with an intake air throttle, the throttle can be used. The transmission gear ratio can be changed to shift the engine to an operating point that produces equal power but contains less oxygen. However, at least in the case of a diesel engine, it is generally necessary to eliminate some of the oxygen in the exhaust by combustion or reforming reactions with reductant that is injected into the exhaust.
The reactions between reductant and oxygen can take place in the LNT, but it is generally preferred for the reactions to occur in a catalyst upstream of the LNT, whereby the heat of reaction does not cause large temperature increases within the LNT at every regeneration.
Reductant can be injected into the exhaust by the engine or a separate fuel injection device. For example, the engine can inject extra fuel into the exhaust within one or more cylinders prior to expelling the exhaust. Alternatively, or in addition, reductant can be injected into the exhaust downstream of the engine.
U.S. Pat. No. 6,832,473 describes a system wherein the reductant is reformate produced outside the exhaust stream and injected into the exhaust as needed. During desulfations, the reformate is injected upstream of an oxidation catalyst. Heat generated by combustion of the reformate over the oxidation catalyst is carried by the exhaust to the LNT and raises the LNT to desulfations temperatures.
U.S. Pat. Pub. No. 2004/0050037 (hereinafter “the '037 application”) describes an exhaust treatment system with a fuel reformer placed in the exhaust line upstream of a LNT. The reformer includes both oxidation and reforming catalysts. The reformer both removes excess oxygen and converts the diesel fuel reductant into more reactive reformate. For desulfations, heat produced by the reformer is used to raise the LNT to desulfations temperatures.
WO 2004/090296 describes a system wherein there is an inline reformer upstream of a LNT and a SCR catalyst. The reformer has a high thermal mass and is intended to operate at exhaust gas temperatures.
The operation of an inline reformer can be modeled in terms of the following three reactions:0.684CH1.85+O2→0.684CO2+0.632H2O  (1)0.316CH1.85+0.316H2O→0.316CO+0.608H2  (2)0.316CO+0.316H2O→0.316CO2+0.316H2  (3)wherein CH1.85 represents an exemplary reductant, such as diesel fuel, with a 1.85 ratio between carbon and hydrogen. Equation (1) is exothermic complete combustion by which oxygen is consumed. Equation (2) is endothermic steam reforming. Equation (3) is the water gas shift reaction, which is comparatively thermal neutral and is not of great importance in the present disclosure, as both CO and H2 are effective for regeneration.
The inline reformer of the '037 application is designed to be rapidly heated and to then catalyze steam reforming. Temperatures from about 500 to about 700° C. are said to be required for effective reformate production by this reformer. These temperatures are substantially higher than typical diesel exhaust temperatures. The reformer is heated by injecting fuel at a rate that leaves the exhaust lean, whereby Reaction (1) takes place. After warm up, the fuel injection rate is increased to provide a rich exhaust. Depending on such factors as the exhaust oxygen concentration, the fuel injection rate, and the exhaust temperature, the reformer tends to either heat or cool as reformate is produced.
In theory, the temperature of the reformer can be controlled through the fuel injection rate. For example if the reformer is heating, the fuel injection rate can be increased to increase the extent of Reaction (2) (endothermic steam reforming) while the extent of Reaction (1) (exothermic complete combustion), which is limited by the exhaust oxygen concentration, remains essentially constant. In practice, this approach often cannot be used. The size and catalyst loading of the reformer are limited for economic reasons, among others, and the efficiency of the fuel reformer is generally insufficient to accommodate high fuel injection rates. As a result, the reformer tends to heat as reformate is being produced, particularly when exhaust oxygen concentrations are in the 8-15% range.
One approach suggested by the '037 application for controlling this heating is to pulse the fuel injection. The reformer is allowed to cool between pulses. In this manner, the reformer can be kept at a desired temperature while a LNT regeneration completes. A desired temperature is, for example, 600° C.±50° C.
In spite of advances, there continues to be a long felt need for an affordable and reliable exhaust treatment system that is durable, has a manageable operating cost (including fuel penalty), and is practical for reducing NOx emissions from diesel engines to a satisfactory extent in the sense of meeting U.S. Environmental Protection Agency (EPA) regulations effective in 2010 and other such regulations.